The present invention relates to an adjustment device for a vehicle seat and more particularly, to a tilt adjustment device which allows the seat back of a vehicle seat to move relative to the base of the vehicle seat. This tilt device is also referred to as a recline mechanism and generally includes an inner gear and outer gear which are moveable to relative one another in order to provide tilt adjustment of the seat back relative to the seat base. Typically, the inner gear is fitted within the outer gear and is at least one tooth smaller such that when a rotating adjustment element drives an eccentric ring or carrier about an axis causing wedge segments to be dragged and eccentrically wedged around the rotational axis, the outer gear moves radially about an axis. Typically, for each rotation of the carrier and wedge segments, the outer gear is driven approximately ten degrees about the axis. After the seat back is in the preferred position, the inner and outer gears are locked from relative movement due to the wedge segments ensuring that the inner and outer gears stay meshed.
The above described rotation mechanism is commonly referred to as a five-door rotary recline mechanism. The five-door recline mechanism works well for vehicles seats where the seats rearward of the seat with the recline mechanism has a separate door. For example, the five-door recline mechanism is common on the front seats of sedans, SUV's and minivans. However, for some seat locations such as the front seats of coupes, middles seats of station wagons, SUV's and minivans, as well as the rear seats of five-door or hatchback vehicles, it is desirable to at times allow the seat back to pivot freely forward. For example, it may be desirable to fold the seat back down to allow ingress and egress quickly without using the five-door rotary recline mechanism. Typically, recline mechanisms that include these additional features that allow free motion of the seat back relative to the seat base in certain conditions are referred to as three-door rotary recline mechanisms. The three-door recline mechanism generally includes a five-door mechanism described above, modified with minor changes, and an additional latch assembly that engages gear teeth on the outer edge of the outer gear. Generally, the latch assembly allows the seat back to be released from engaging the outer gear of the five-door recline mechanism such that it may rotate independent of the five-door recline mechanism that is included in the three-door recline mechanism. In some embodiments, the free rotation of the vehicle seat back also allows through additional elements for the track to be released and the seat to slide forward in one motion, typically to allow easy ingress and egress from the vehicle seat.
The outer and inner gears of the five-door recline mechanism and also the five-door recline mechanism when used with the modified outer gear in combination with the latch assembly of the three-door rotary recline mechanism are typically stamped out of a mild steel to allow easier forming of the gears. The mild steel allows the metal to easily be formed without becoming brittle or cracking during the various forming steps. In particular, the outer gear includes a hub which extends outwardly from the original plane of the sheet of metal forcing manufacturers to traditionally use only mild steel for the forming of the outer gear. Since mild steel is used in the forming process of the inner and outer gears, manufacturers had to heat treat the gears once the forming process was complete. The heat treating process hardened the gears thereby providing increased life expectancy and improved wear characteristics. Heat treating added additional processing steps that were time-consuming and expensive. The reason that most inner gears and all outer gears are formed out of a mild steel such as 1020, 4130, or 4140 steel is that the higher carbon content of stronger steels cause them to be brittle and during the forming process, and in particular, the forming of the outwardly extending hub, the edges of the hub would crack or shatter on the outer gear. Therefore, manufacturers found it impossible to stamp the outer gear out of a higher quality steel while eliminating the heat treating process.
The rotary recline mechanisms also further include a carrier which pushes a pair of wedges against a bearing located inside of the inner gear. The carrier is rotated forcing the inner gear against the outer gear. The inner gear is configured to have one or more fewer teeth than the outer gear such that when the inner gear is forced into engagement with the outer gear and the carrier spins for each revolution of the carrier, the outer gear is forced approximately ten degrees about the axis of rotation. Certain designs of carriers while providing sufficient wedge capabilities allowed unintended rocking of the seat back as the main point of pressure on the bearing was centered between the two wedges. In addition, in seat assemblies where the power tilt mechanism was frequently used, the design of the carriers and bearings could cause premature failure. Therefore, until now, manufacturers have not been able to provide an anti-rock carrier with improved longevity.